1. Field of the Invention
The present invention relates to an image-forming apparatus of an electrophotographic system or an electrostatic recording system such as a copying machine or a printer, and to an image-forming method to be used for the image-forming apparatus.
2. Related Background Art
Various image-forming apparatuses each intended for forming a color image on a transfer material by means of an electrophotographic recording system have been proposed, and some of them have been put into practical use. (see, for example, Japanese Patent Application Laid-Open No. 2001-183885)
FIG. 7 shows an example of the structure of a color image-forming apparatus adopting an in-line system. FIG. 7 is a side view schematically showing the main internal structure of the apparatus. The image-forming apparatus is constituted as a quadruple photosensitive drum-intermediate transfer type color printer of an in-line system.
The color printer of this in-line system includes an intermediate transfer belt 106. The intermediate transfer belt 106 is suspended by a driver roller 107, a driven roller 109, and a tension roller 108, and is rotated in the direction indicated by an arrow A shown in FIG. 7. Four photosensitive drums 101a to 101d serving as image-bearing members are arranged in series along the intermediate transfer belt 106. The photosensitive-drums-101 and other image-forming means constitute arrangement stations PY, PM, PC, and PK on which image-forming means for forming yellow, magenta, cyan, and black toner images are respectively arranged.
The image-forming means on the arrangement stations PY, PM, PC and PK are respectively constituted by the photosensitive drums 101a to 101d, and charging devices 102a to 102d, exposing devices 103a to 103d, developing devices 104a to 104d, and photosensitive drum cleaners 105a to 105d arranged around the corresponding photosensitive drums. The image-forming means for the respective colors have substantially the same structure except that yellow toner, magenta toner, cyan toner and black toner are stored in the developing devices 104a to 104d, respectively.
The operation of forming a full-color (four-color) image will be described. At first, each of the photosensitive drums 101 rotates, and its surface is uniformly charged by the corresponding one of the charging devices 102. Next, each of the exposing devices 103 irradiates a laser beam modulated in accordance with image data, so a desired electrostatic latent image corresponding to each color is formed on the surface of each of the photosensitive drums 101. The electrostatic latent images on the respective photosensitive drums 101 are developed at developing positions by the respective developing devices 104 with colored toners so as to be visualized as yellow, magenta, cyan, and black toner images, respectively.
The toner images of the respective colors formed on the photosensitive drums 101 are electrostatically transferred onto the intermediate transfer belt 106 at respective transfer nip portions opposed to the photosensitive drums 101 by transfer rollers 110 of transferring means in such a manner that the toner images are sequentially superimposed on each other. A transfer material P is fed from sheet-feeding means to a secondary transfer nip portion between the intermediate transfer belt 106 and a secondary transfer roller 112 via conveying means, and then the toner images on the intermediate transfer belt 106 are electrostatically and collectively transferred onto the transfer material P.
Residual toner on the photosensitive drums 101 after the transfer is removed by the photosensitive drum cleaners 105 each equipped with a cleaning blade or the like so as to be ready for a next image-forming step.
Residual toner on the intermediate transfer belt 106 after the secondary transfer is removed by an intermediate transfer belt cleaner 111 so as to be ready for a next image-forming step.
As shown in FIG. 8, the toner images of the four colors are collectively transferred onto the transfer material P as described above to be formed on the transfer material P. In FIG. 8, M denotes magenta toner, C denotes cyan toner, Y denotes yellow toner, and K denotes black toner.
Here, a toner layer is pressurized and heated in a fixing nip by a fixing roller 126 and a pressure roller 127 in a fixing unit 125 shown in FIG. 7 to be fixed on the transfer material P.
At this time, in the case where the storage elastic modulus G′(T) at a saturated temperature (T) in the fixing nip of each toner at the time of a fixing step is low, the excessive impregnation of the toner layer into the transfer material may occur as shown in FIG. 9 when the toner layer is pressurized and heated. As a result, as shown in FIG. 10, a paper fiber appears on the surface of an image, so the uniformity of the gloss value of the surface of the image is lost and an image density reduces. Thus, there may arise a problem in that a desired image cannot be obtained.
In view of the foregoing, attempts have been made to increase the storage elastic modulus G′(T) at a saturated temperature in a fixing nip of each toner at the time of a fixing step by means of a method involving reducing a saturated temperature in the fixing nip at the time of the fixing step or changing the kind of the toner itself. In this method, however, toner insufficiently melts to reduce the gloss value of the surface of an image or to generate a factor of the deterioration of the fixability of a toner image on a transfer material. As a result, there may arise a problem in that a desired image cannot be obtained.